Thompson, William (1805–52), naturalist, was born 2 November 1805, probably in Donegall Square, Belfast, eldest son of William Thompson (d. 1819), a prosperous linen merchant who owned a bleach-green at Wolf Hill, outside the town, and Elizabeth Thompson (née Callwell). He had at least two older sisters and several younger brothers. His mother's father was Robert Callwell (1764–1838), a printer, book-collector, partner in the Commercial Bank, Belfast, and one of the owners of the Northern Star newspaper, whose wife was a Magee of Newbridge, related to William Magee (qv) (d. 1827) and John Magee (qv) (d. 1809).
After attending the RBAI from 1818, William was apprenticed (1821) in the linen business of William Sinclair (qv). When his apprenticeship ended, Thompson went with his cousin George Langtry, later a wealthy shipowner, on a four-month tour of the Low Countries, the Rhine, Switzerland, and Italy (May–September 1826). On his return to Belfast he set up his own business in linen bleaching. Despite early success, losses were incurred; and as family and economic circumstances changed, Thompson increasingly concentrated on his natural history studies. By 1831 he had given up business. A self-taught naturalist, related by ties of kinship or friendship to most of the liberal and cultivated families of the ‘northern Athens’, he was shy and fastidious, but was persuaded in 1826 to join the Natural History Society of Belfast by its founder, his friend James Lawson Drummond (qv). He read his first paper to the society, ‘On the birds of the Copeland Islands’ on 13 August 1827. In that year he became a member of the Belfast Natural History Society's council, and in 1833 he was chosen as one of the society's vice-presidents; he was president from 1843 until his death.
Thompson became the most important naturalist in mid nineteenth-century Ireland. From 1827 to 1852 he contributed almost eighty papers on Irish natural history to the Magazine of Botany and Zoology and the Proceedings of the Zoological Society of London. From 1836 to 1851 he contributed to the Magazine of Natural History. Invited to travel to the Levant and the Aegean Sea in April–July 1841 with Edward Forbes, professor of natural history at the University of Edinburgh, on HMS Beacon, Thompson observed twenty-three species of birds on migratory flights, and published ‘Notice of migratory birds’ in Annals of Natural History. His authoritative observations added considerably to knowledge of the still-to-be-ascertained details of migratory patterns; indeed, some people refused to believe, even at that date, that birds did migrate. Thompson published other papers in the same journal during 1841–3. At a meeting of the British Association in Glasgow in 1840 his ‘Report on the fauna of Ireland – Division Vertebrata’ attracted favourable notice. He presented and published a second and final part enumerating the invertebrates at the Cork meeting of the British Association in August 1843. The two reports formed the most complete catalogue of Irish fauna yet published. Thanks to an assiduous correspondence with a network of informants, as well as his own extensive observations, Thompson added several hundred species to Irish fauna lists, perhaps more than 800.
Thompson's chief work, The natural history of Ireland, became the standard text in Irish zoology in the nineteenth century. The first three volumes, published between 1849 and 1851, dealt with birds, particularly their habits and habitats rather than physical descriptions; he was one of the first naturalists to note the effects of industrialisation and other human activities on birdlife. He left instructions for his manuscripts on the remaining vertebrates and all the invertebrates to be prepared for publication by Robert Patterson (qv) and James R. Garrett. Robert Ball (qv) and George Dickie (qv) also assisted. (Dickie's Flora of Ulster (1864) contains observations supplied by Thompson). Thompson's notes, though detailed and comprehensive, all required checking, and were found on tiny scraps of paper, even scribbled on the flaps of old envelopes. James Thompson of Macedon, Belfast, painstakingly gummed them all into blank notebooks to facilitate the work of his brother's literary executors, who prefaced the posthumous publication in 1856 with a lengthy memoir of their friend.
From about 1820 to 1852 William Thompson lived with his mother at 1 Donegall Square West, Belfast, commuting from Holywood House, Holywood, Co. Down, during the summer. His daily routine began with research, correspondence, or writing for publications for four hours after breakfast. After a two- or three-hour exercise period and dinner, he returned to work for a further two to three hours. He was president of the Belfast Literary Society (1837–9) and also an enthusiastic patron of the visual arts in the city. He enjoyed hunting, wildfowling, shooting in Scotland, and gardening, though his health deteriorated from the 1840s.
Early in 1852 he travelled to London to make arrangements for that year's Belfast meeting of the British Association. On 15 February he became ill, having suffered a minor stroke. He died, unmarried, at his Jermyn St. lodgings on the day he was due to return home, 17 February 1852. He was buried in Clifton St. graveyard, Belfast. Thompson bequeathed his collection to the Belfast Natural History Society, and in March 1852 the Society added a memorial Thompson Room to its museum, paid for by subscription.
Thompson was a corresponding member of natural history societies in Boston and Philadelphia and had many friends; he is known to have assisted many other researchers in Ireland, Britain, and the Continent. One of those who thought highly of his work was Charles Darwin, with whom he had corresponded. Thompson also helped many local people, including the poet Francis Davis (qv), with money and practical assistance. He was much loved, and his friends were deeply saddened by his death. His niece Sydney Mary Thompson, later known by her married surname, Christen (1847–1923), who was born in Belfast, was an amateur naturalist, geologist, and artist, one of the first women to achieve distinction in geology.